There has been recent interest in powering vehicles with electricity that is created on board by electrochemical reaction of hydrogen and oxygen within a polymer electrolyte membrane to produce water as a by product. In a polymer electrolyte membrane fuel cell, also called a proton exchange membrane, hydrogen is passed on the anode side of the membrane while air, containing the oxygen, is passed on the cathode side of the membrane. At the anode, a platinum catalyst causes the hydrogen to ionize into hydrogen ions or more particularly, protons and electrons. The polymer electrolyte membrane allows only the protons to pass through it to the cathode side where the hydrogen combines with the oxygen to produce the water. The electrons pass from the anode to the cathode through an external circuit that can be connected to an electrical motor to propel the vehicle.
The hydrogen purity requirement for the polymer electrolyte membrane fuel cell is quite stringent because any carbon monoxide, as well as other impurities in the hydrogen, will tend to poison the platinum catalyst used in the fuel cell. In the prior art, it has been proposed to generate the hydrogen on board the vehicle, purify the hydrogen with a palladium based membrane system and then supply the hydrogen, once purified, to the polymer electrolyte fuel cell. An example of such a system can be found in U.S. Pat. No. 6,348,278 in which a fuel is reformed in a reactor and the reformed hydrogen is purified in a downstream palladium based membrane system. The palladium based membrane system is capable of separating the hydrogen when such system is heated to a high operational temperature. However, given that the reformed hydrogen is produced at a high temperature, the reformed hydrogen itself can be used for such heating purposes. The hydrogen permeate is supplied to the fuel cell and part of the retentate is combusted in a reactor utilizing an oxidation catalyst to generate heat that is used in preheating the feed to the reformer.
In other types of systems to be used in connection with vehicles the hydrogen is produced in a large scale steam methane reforming system that produces a synthesis gas that is purified to produce the hydrogen. The hydrogen is then distributed to fueling stations where it is supplied to an on board high pressure tank contained in the vehicle either in a gaseous or liquid form. However, even in such case, high purity hydrogen has to be supplied to an on board high pressure tank contained within the vehicle. As can be appreciated, as the purity of the hydrogen increases, the cost of the hydrogen as a fuel for the vehicle also increases. Moreover, the problem is only exacerbated in that in transporting and charging the vehicle fuel tank with the hydrogen, impurities can be introduced into the hydrogen. This does not allow the use of standard grade hydrogen having a purity of at least about 99.95 percent by volume. Typically, even where high purity hydrogen is used, because such impurities have a higher volatility than the hydrogen, hydrogen vapor containing the impurities is continually being vented from the on board vehicle tank. Hydrogen transport membranes of the type containing palladium are not used for such purification purposes given that there is no readily available heat source contained in the vehicle that can be used for such purposes.
As will be discussed, the present invention allows the use of a hydrogen transport membrane to remove impurities from the hydrogen to be supplied to the fuel cell from a fuel tank mounted in a vehicle and in particular, the use of standard grade hydrogen in the first instance.